1. Field of the invention The present invention relates to a high strength coil spring made of an oil-tempered steel wire, and more Particularly to a manufacturing method of the coil spring capable of restraining delayed fracture of the coil spring.
2. Description of the Prior Art
In recent years, it is required to provide lightweight valve springs adapted for use in automotive engines. To satisfy such requirements, there have been proposed various methods for strengthening an oil-tempered steel wire for the valve springs. For example, there has been proposed an oil-tempered steel wire with tensile strength of more than 210 kgf/mm.sup.2 and internal hardness of more than Hv 550, which contains 0.45 to 0.8% C, 1.2 to 2.5% Si. 0.5 to 1.5% Mn and 0.5 to 2.0% Cr, by weight and at least one metallic element selected from the group of 0.1 to 0.7% Mo, 0.05 to 0.6% V, 0.2 to 2.0% Ni and 0.01 to 0.2% Nb, by weight and contains Fe and impurity elements as a remainder.
In the oil-tempered steel wire of this kind, it has been found that there occur breakage of the steel wire during a cold coiling process and delayed fracture of the steel wire after the cold coiling process. Disclosed in Japanese Patent Laid-open Publication No. 4(1992)-285142 is a method of decarburizing the surface of the steel wire for preventing the steel wire from breakage during the cold coiling process. The surface hardness of the steel wire defined by decarburizing treatment prior to the oil-tempering process is, however, limited to less than Hv 400. For this reason, the effect of the nitriding treatment for increasing the surface hardness of the steel wire is reduced, resulting in decrease of fatigue strength of the valve springs. In addition, for increasing the surface hardness of the steel wire more than Hv 900 by nitriding treatment in an atmosphere of ammonia gas, it is required to carry out the nitriding treatment at 500 .degree. C. for more than six hours. This lowers the productivity of the steel wire. Furthermore, in the oil-tempered steel wire described above, delayed fracture of the steel wire will occur after the coiling process due to an increase of retained austenite and an increase of residual stress on the surface of the steel wire caused by the coiling process.